Studies on the generation of polyaniline microstructures using microemulsion polymerization

Abstract

Molecular self-assembly has the potential to enable the fabrication of technologically useful polymer microstructures [1±8]. As part of our research programme [9±11] to design methods of controlling the microstructural evolution in conjugated polymers, we report here the synthesis conditions for well-de®ned polyaniline (PANI) microstructures using surfactant assemblies (in microemulsions) as structure-directing ` templates''. It is well known [12] that amphiphiles assume different aggregate shapes depending on the microemulsion=micellar composition, expressed through water solubilization factor, w0 and packing parameter, p [13, 14]. We have used this concept recently to demonstrate its ability to improve crystalline order and well-de®ned microstructures [9±11], and now show here the possible mechanism of evolution of PANI microstructures obtained via controlled microemulsion polymerization [10]. Prior to chemical polymerization, aniline was distilled twice. Anionic surfactants, sodium dodecylsulphate (SDS) and sodium bis-2-ethylhexylsulphosuccinate (aerosol OT or AOT) were used as-received from Sigma. PANI was chemically synthesized by mixing two separate parts of water-in-oil (w=o) microemulsion, containing monomer and oxidant, respectively, and microemulsions were formulated taking w0 as a variable for every polymerization reaction to study its in uence on the polymer microstructures. Table I shows four different microemulsion systems using anionic surfactants, which lead to the formation of different microstructures. On examination of Fourier transform infrared (FTIR) spectra of PANI samples, it was found that PANI particles are surfactant-stabilized [11]. We worked on the premise that the formation of these microstructures is the result of adsorption of the cationic ` nascent'' polyaniline chain to the anionic surfactant aggregates. Thus, electrostatic attraction undoubtedly contributes to the adsorptive interaction between the surfactant microstructures and the nascent polymer. We have reported [10] earlier the scanning electron micrographs (SEM) of needles and akes of chemically synthesized PANI powders. Table II shows the in uence of water solubilization factor w0 on the evolution of microstructures. Water solubilization factor or ` water-pool'' ratio w0 is